1. Field of the Invention
The present invention relates generally to a pressure regulator valve for controlling the level of the line pressure which is used to engage the friction elements of an automatic automotive transmission and more specifically to a pressure regulator valve which can regulate the level of the line pressure in a manner wherein a manually induced downshift from D range for the purposes of obtaining engine braking and the like, does not induce severe shift shock and wherein adequate friction element engagement under modes wherein a relatively large amount of torque is required to be transmitted through the transmission is obtained.
2. Description of the Prior Art
JP-A-60-1444 discloses a previously proposed line pressure control arrangement wherein when the transmission is conditioned to produce the D range the level of the line pressure is changed in accordance with the level of the throttle pressure and when the transmission is conditioned to produce the manual 2nd speed range the level of the line pressure is held at a predetermined level and unaffected by the changes in throttle pressure.
The reason for this control is that when the transmission is shifted into manual 2nd speed range to produce engine braking the level of throttle pressure reduces to zero and the level of line pressure tends to fall to its minimum level. Accordingly, the pressure which is supplied to the friction elements which are engaged during this mode is insufficient and slippage tends to occur. For this reason it is necessary to set an arbitary predetermined limit of the line pressure in order to enable the friction elements to transmit the required amount of torque.
However, this arrangement has suffered from the drawback that while the predetermined level is sufficient for engine braking (which occurs with a zero throttle pressure level) in the event that the engine throttle valve is moved to a wide open position the amount of torque which needs to be transmitted exceeds that which can be transmitted by the friction elements engaged with said predetermined pressure level.
If the level is raised to a relatively high value in order to compensate for the above shortcomming, when the transmission is shifted from D range to the manual 2nd speed range, the high pressure level which is induced tends to produce a strong shift shock.